Fin materials to be used for automobile heat exchangers, such as radiators, by brazing, are formed into corrugated shapes, and are assembled with tube materials, and are then bonded by brazing. Needs for light weight and cost reduction of heat exchangers are ever-increasing in recent years, and thinning of major members, including the fin material, is advancing further. To maintain or improve characteristics of the heat exchanger when the fin material is thinned, various elements have been added to the fin material, or the manufacturing process has been studied, in recent years, to enhance the mechanical strength of the fin material.
As examples for changing elements to be added, fin materials of Al—Fe—Ni-series alloys are proposed (see, for example, JP-A-7-216485 (“JP-A” means unexamined published Japanese patent application) and JP-A-8-104934). However, since the fin materials described in these publications are poor in self-corrosion resistance, the materials are alloys not suitable to be made into a thin fin, although they are excellent in mechanical strength and heat conductivity. As examples of a production process studied, there are proposed fin materials of Al—Fe—Mn—Si-series alloys, for enhancing the mechanical strength and electrical conductivity, by specifying the cooling rate in a continuous casting and rolling process (see, for example, International Patent Application Publication No. WO00/05426). However, as described in WO00/05426, the recrystallized grain diameter of the raw material for this fin material is extremely small. Due to the above extremely small size, the resulting fin material may often be buckled by diffusion of a filler alloy element(s) during brazing, and the material is not suitable to be made into a thin fin.
Further, there is proposed a fin material having high strength and high heat conductivity, by using twin-roll continuous casting and rolling (see, for example, JP-A-2002-241910). Resistance to diffusion of the filler alloy is enhanced in this fin material by maintaining a rolled texture or structure (a fibrous structure) until heating at near a brazing temperature. However, because the amount of spring back is so large in recently developed highly strengthened and thinned fin materials, a desired fin pitch cannot be obtained by forming into a corrugated shape in some cases.
Accordingly, it is proposed to recrystallize aluminum alloys by intermediate annealing to reduce the amount of spring back, thereby to reduce proof stress of the material. However, the bonding ratio by brazing may be reduced in this case, since the filler alloy is diffused as described above when the recrystallized structure is fine, or, on the contrary, the peak height of the corrugated fin (the height from an R portion at a trough to an R portion at the neighboring peak of the corrugated fin) becomes irregular when the recrystallized structure is large in a certain extent. Irregularity of the height of the fin peak will be described in detail in below.
Further, there are proposed fin materials excellent in mechanical strength after brazing; heat conductivity, self-corrosion resistance, and erosion resistance, but nothing is mentioned about formability into corrugated shape (see, for example, JP-A-2002-256402). That is, although a final cold-rolling ratio is 15 to 50% in the claim in the patent publication above, it is apparent that the strength of the material and the configuration of the crystal structure at a final cold-rolling ratio of 15% are largely different from those at a final cold-rolling ratio of 50%. This is because the formability into a corrugated fin shape has not been taken into account. In addition, continuous annealing of less than one minute is used in the intermediate annealing process in each of the examples in JP-A-2002-256402. Although continuous annealing is used up to a sheet thickness of 0.11 mm when the process is counted back from the final sheet thickness and final cold-rolling ratio, this process may be considered to be a quite difficult process using conventional industrial facilities, and only a limited number of facilities can conduct it. Contrary, in a usual continuous annealing furnace, it is assumed that annealing is applied in a range of sheet thickness of 0.3 to 1.0 mm, from the viewpoint of cost and performance. For example, when a sheet of thickness 0.3 mm is continuously annealed, followed by cold rolling to 0.08 mm, the final cold-rolling ratio exceeds 70%, and it is quite highly possible to cause spring back during a process for forming corrugated fins, and erosion may occur in the brazing process.
Other and further objects, features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.